CODEWITHSUNDEEP
c
jfowkes
jfowkes

Reputation: 1565

Does C support anonymous members of unions?

I have a union that is an overlay of an array on three floats:

union {
    float currents[3];
    struct {
        float run;
        float standby;
        float sleep;
    };
} MyCurrents;

run can be accessed either by MyCurrents.run or MyCurrents.currents[0].

Is there a way to have the currents array anonymous, such that I can access run by simply using MyCurrents[0]? Having the . accessor seems a bit redundant in this use case.

I realise I can use ((float*)&MyCurrents)[0] but that's horrible and I'm not sure it's actually a reliable method to use.

Upvotes: 1

Views: 101

Answers (1)

Eric Postpischil
Eric Postpischil

Reputation: 222526

Per the C standard, members that are structures or unions may be anonymous. This works because the members within them are not anonymous, so every subobject within the enclosing structure or union has a name. Anonymous members of other types are not supported (except that bit fields used for padding may be anonymous, but they are also not normally accessible).

(The C grammar allows you to include declarations without names (C 2018 6.7.2.1 1: The struct-declarator-list is optional in a struct-declaration), but they do not create members, and there would be no way of referring to them. And declaring a member without a name requires omitting the entire declarator of the grammar, which includes (per 6.7. 1) * for pointers, [ and ] for arrays, and ( and ) for functions. So the grammar would permit you to declare struct { float; float f; } foo; but not struct { float [3]; float f; } foo;.)

Upvotes: 3

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